COMPARISON OF OUTCROP AND SUBSURFACE SANDSTONE PERMEABILITY DISTRIBUTION LOWER CRETACEOUS FALL RIVER FORMATION, SOUTH-DAKOTA AND WYOMING

Quantification of permeability structure in outcrop reservoir analogs documents the distribution of how units and barriers important for res ervoir management and simulation. Differences in burial history and di agenesis of the outcrop analog and the subsurface reservoir must be qu antified, however, before outcrop permeability data can be used to mod el subsurface strata appropriately. A case study of the Lower Cretaceo us Pall River Formation shows that permeability differences between fa cies are accentuated by diagenesis, and permeability variation increas es with the extent of burial diagenesis, The Pall River Formation, whi ch is exposed in outcrop around the Black Hills uplift in Wyoming and South Dakota, USA, produces oil from fluvial and estuarine valley-fill sandstones in the adjacent Powder River Basin. Outcrop sandstones wer e buried to 2 km before being uplifted at the end of the Cretaceous, w hereas reservoir sandstones remained at depths of 2-4 km throughout th e Tertiary, The permeability of outcropping Pall River sandstones has a lognormal distribution. The main controls on permeability in outcrop sandstones are (1) ductile-grain content, (2) grain size, both proper ties being controlled by the energy of the depositional environment, a nd (3) hematite cement, an uplift-related diagenetic feature not prese nt in the subsurface. Hematite was preferentially precipitated along z ones of permeability contrast, including sequence boundaries, complica ting the determination of changes in primary depositional permeability at important flow-unit boundaries. In contrast, the permeability of P all River reservoir sandstones at a depth of 4 km has a bimodal distri bution and is controlled by ductile-grain content, grain size, and qua rtz-cement volume. Quartz cementation and compaction due to ductile-gr ain deformation were more extensive in the estuarine facies, resulting in a greater permeability reduction during burial diagenesis than was observed in fluvial sandstones. Abundant stylolites developed in sand stones with clay partings or mud clasts, features that are common in e stuarine sandstones. The stylolites acted as an internal source of sil ica. Sandstones near the stylolites typically have low permeability th at probably resulted from above-average volumes of quartz cement. Diag enesis increased the coefficient of variation of permeability in all f acies in the subsurface. This diagenetic overprint should be considere d if outcrop permeability data are used to model a subsurface reservoi r having a very different burial history.